Business and transportation leaders reveled in news of the federal government's conditional approval of Dulles rail yesterday, saying the decision will revive an ambitious effort to shape growth and improve the quality of life in the Washington region.

"The Bible describes faith as the substance of things hoped for. This certainly seems to fit in that category," said Clark Tyler, chairman of the Tysons Land Use Task Force, which has spent years planning for development around rail. "We had faith that Metro was coming, and it looks like it's going to."

The decision by the Federal Transit Administration to advance the project comes three months after federal officials expressed serious concerns that threatened the project's future. Yesterday, Transportation Secretary Mary Peters said "much work remains to be done . . . and there is no guarantee" of full federal funding. But rail backers focused on the thrust of Peters's decision, which essentially allowed the rail project to go forward.

"We're all celebrating today," said Eileen Curtis, president of the Dulles Regional Chamber of Commerce. Curtis said the "headaches we gave some people with our robust fight back" played a role in getting federal support.

Curtis said that for weeks the group sent 1,000 e-mails daily, calling on members to pressure federal and state officials.

James C. Dinegar, president and chief executive of the Greater Washington Board of Trade, said the rail project will be a powerful economic stimulus for the area.

"This is a region-wide real shot in the arm," he said. "At $5 billion over the next several years, it will also help smooth out some of the dips in the economy. That's a pretty good shot of money."

Dinegar said he and his members reached out aggressively to senior Bush administration officials. "We virtually melted Josh Bolten's and Ed Gillespie's fax machines, we were told," he said. Bolten is the White House chief of staff, and Gillespie, a former top GOP official who also was chairman of the Republican Party in Virginia, is an adviser to President Bush.

Extending rail to Dulles is key to shaping growth, addressing traffic and improving the quality of life in the corridor, supporters said. Plans for remaking Tysons Corner into a walkable downtown easily reachable by mass transit, and longer-term efforts to improve transportation throughout Northern Virginia, are back on course, they said.

The decision comes after the Virginia Supreme Court junked a hard-fought transportation funding compromise that would have raised hundreds of millions of dollars for much-needed transportation projects in the region, ruling the tax collection process unconstitutional.

"After such a spate of bad news, in terms of transportation funding, the reversal of the decision is wondrous," said John Townsend, a spokesman for AAA Mid-Atlantic.

Critics of the rail proposal, however, were disappointed.

"It's the greatest tragedy for taxpayers in a long time," said state Sen. Ken Cuccinelli II (R-Fairfax). "It's going to suck down every available transportation dollar that comes within its gravitational pull."

Although the federal decision yesterday took many by surprise, some project supporters had been prescient about the development.

John Schlichting, managing director of development for the JBG Cos., a Chevy Chase-based real estate firm that has made vast investments in properties along the proposed Metro line in Tysons and Reston, said in January that the rail project has such national importance that he refused to entertain the notion that it could die.

"We're not surprised at all," Schlichting said yesterday. "We're very happy about it, I'll tell you that." He said he did not know what had swung the balance. Schlichting also declined to say precisely how much his company has invested in the area but said it is "in the hundreds of millions."

An elaborate maypole dance and ribbon-cutting ceremony is scheduled today for a Westin hotel in Schlichting's development in Reston.

"We've assembled that 35 acres over the last eight years," he said, referring to the Reston Heights development, where the hotel is opening. "All along, it's been in anticipation of rail coming to Dulles."